1. Field of the Invention
The present invention relates to a ferroelectric thin film device and a method of producing the same, and more particularly to a ferroelectric thin film device applicable to a capacitor for use in a DRAM or a ferroelectric RAM (hereinafter referred to as a FeRAM), a pyroelectric device, a micro actuator, a thin film capacitor, a small-sized piezoelectric device, and the like and to a method of producing such a ferroelectric thin film device.
2. Description of the Related Art
In recent years, there have been intensive studies on thin films of perovskite compounds of Pb or other elements such as BaTiO.sub.3 (hereinafter referred to simply as BTO), SrTiO.sub.3 (hereinafter referred to simply as STO), (Ba, Sr)TiO.sub.3 (hereinafter referred to simply as BSTO), PbTiO.sub.3, (Pb, La)TiO.sub.3 (hereinafter referred to simply as PLT), PZT, PLZT, and Pb(Mg, Nb)O.sub.3 (hereinafter referred to simply as PMN), formed on an Si substrate. In particular, there is a great demand to grow an epitaxial thin film of a Pb-based perovskite compound such as PZT or PLZT having the property of great residual dielectric polarization thereby achieving spontaneous polarization aligned in one direction and thus achieving a greater polarization value and better switching characteristics. Such an epitaxial thin film will be extensively used as a high-density storage medium in various applications, and thus the development of a technique of growing such an epitaxial thin film is urgent.
In applications in which spontaneous polarization is aligned in one direction along the film thickness direction, it is required that a layer structure called an MFM (metal-ferroelectric-metal) structure consisting of a ferroelectric thin film sandwiched by electrically conductive layers (electrode layers) be formed on an Si substrate. In conventional techniques, however, it is difficult to form a triaxially-oriented ferroelectric oxide thin film having good crystal quality for the reasons described below.
First, when the conductive film on the Si substrate is realized using a metal film such as Ag, Au, or the like, oxidation occurs at the interface between the metal film and the ferroelectric oxide thin film and interdiffusion occurs between the metal film and the underlayer material, that is, the Si.
Second, when the metal film is formed using Pt, it is impossible to grow an epitaxial Pt film directly on an Si substrate although an epitaxial Pt film can be formed on an oxide single-crystal substrate such as MgO or SrTiO.sub.3.
Third, when the conductive thin film is formed using an oxide such as (La, Sr)CoO.sub.3 (hereinafter referred to as LSCO), it is necessary to form another layer between the Si substrate and the LSCO layer. A specific example of such a structure is PLZT/LSCO/BiTO/YSZ/Si. In this structure, however, it is difficult to form a high-quality epitaxial ferroelectric layer at the top of the structure. (BiTO is a short designation for Bi.sub.4 Ti.sub.3 O.sub.12 and YSZ is a short designation for ZrO.sub.2 doped with Y (yttrium)).
Fourth, with only one exception, no one has succeeded in growing a thin epitaxial layer of PZT, which is a Pb-based perovskite oxide ferroelectric, on an Si substrate. The exception is Koinuma et al. (Jpn. J. Appl. Phys. Vol. 135 (1996), L574) who grew an epitaxial layer of TiN on an Si substrate by means of a pulsed laser deposition (PLD) process at a low pressure (less than 10.sup.-7 Torr) and then formed an SrTiO.sub.3 (STO) buffer layer thereon, and finally formed an epitaxial thin film of PZT at the top, wherein the STO layer on the TiN film was formed in the ambient vacuum at a pressure of 10.sup.-5 Torr (at 550.degree. C.) which was quite different from the ambient (flow of .degree. 2 gas at 0.1 Torr at 450.degree. C.) employed to form the PZT layer. In this technique, oxidation of the TiN layer was prevented by growing the STO with the perovskite phase at the ambient with an extremely low oxygen partial pressure.
The Koinuma et al technique has the following-problems. It is necessary to employ an extremely low pressure of less than 10.sup.-7 Torr when an epitaxial thin film of TiN is grown on an Si substrate. To achieve such an extremely low pressure, it is necessary to use a high- quality vacuum system including a deposition chamber. Such a high-quality vacuum system is very expensive. Another problem of this technique is that to obtain a well-oriented PZT film, a buffer layer of STO, which behaves as a paraelectric at room temperature, is required between TiN and PZT layers.
In the conventional techniques, as described above, it is difficult to form a ferroelectric thin film of an oxide with a perovskite structure directly on an epitaxial layer serving as an electrode layer formed on an Si single-crystal substrate. In particular, it is very difficult to form a ferroelectric thin film of a Pb-based perovskite oxide in such a manner as to be oriented to a high degree (uniaxially or to a high degree).